A process for purifying a feed gas mixture with respect to its less strongly adsorbed component which integrates temperature swing adsorption (TSA) and pressure swing adsorption (PSA) to optimize overall performance is taught in the art. Specifically, U.S. Pat. No. 4,770,676 by Sircar et al. teaches such a process in the context of purifying a landfill feed gas mixture with respect to it methane component. Sircar's process comprises:
(a) passing the feed gas mixture through a temperature swing adsorption bed containing an adsorbent selective for the retention of the strongly adsorbed component to produce an adsorption bed saturated with the strongly adsorbed component and a temperature swing effluent stream enriched in the moderately adsorbed component and the less strongly adsorbed component (the strongly adsorbed component will generally comprise the species water, C.sub.4 +hydrocarbons and sulfur-containing compounds; the moderately adsorbed component will generally comprise the species carbon dioxide and C.sub.3 hydrocarbons; and the less strongly adsorbed component will generally comprise the species methane and C.sub.2 hydrocarbons);
(b) regenerating the temperature swing adsorption bed via:
(1) heating the TSA bed by heating a first regeneration gas and subsequently passing the heated first regeneration gas through the TSA bed in order to vaporize and desorb at least a portion of the strongly adsorbed component from the TSA bed; and PA1 (2) cooling the TSA bed to the temperature of the TSA bed in step (a) by passing a second regeneration gas through the TSA bed; PA1 (1) depressurizing the PSA bed to produce a depressurization effluent stream enriched in the moderately strongly adsorbed component wherein a first portion of said depressurization effluent stream is used as at least a portion of the first regeneration gas in step (b) (1) and wherein a second portion of said depressurization effluent stream is used as at least a portion of the second regeneration gas in step (b) (2); and PA1 (2) repressurizing the PSA bed to the pressure of the PSA bed in step (c) with a third regeneration gas.
(c) passing the temperature swing effluent stream through a pressure swing adsorption bed containing an adsorbent selective for the retention of the moderately adsorbed component to produce an adsorption bed saturated with the moderately adsorbed component and a pressure swing effluent stream,enriched in the less strongly adsorbed product component;
(d) regenerating the pressure swing adsorption bed via:
There is a problem with Sircar, however, when the less adsorbable component of the feed gas mixture contains significant quantities of a species which is not desired in the product stream such as nitrogen. The problem is that such a species will not be removed by Sircar's process and thus contaminates the product produced in step (c). Furthermore, Sircar does not produce his product in the liquid state which is often required where the product is to be transported and/or used as a liquid fuel. The present invention overcomes these problems by integrating a cryogenic distillation section into Sircar.
An important application of the present invention is the purification and liquefaction of a natural gas feed stream with respect to its methane/C.sub.2 hydrocarbon component. This application is important because, as the awareness of the benefits from clean air increases, there is a trend towards replacing petroleum fuels by liquid methane in the transportation industry. Although the United States has an abundance of natural gas, it contains impurities such as water, sulfur-containing compounds, light hydrocarbons (ie C.sub.3 hydrocarbons; note that C.sub.2 hydrocarbons are generally not considered an impurity), heavy hydrocarbons (ie C.sub.4 +hydrocarbons) and carbon dioxide which have to be removed prior to liquefaction to obtain the liquid methane/C.sub.2 hydrocarbon product.